Chaostale: A Tale of the Underground
by Aurlim Lapalim
Summary: In the middle of a fight against Eggman, Sonic and Shadow fall through a portal, ending into the Underground and suffering a transformation in the process. Without Frisk around in the monster's world, this gives the chance for any creature to take control over the timeline... "Golly! It looks like things will get a little more interesting around here!"


**... I'm just adding another story to both fandoms, while trying to get motivated to keep writing stuff again, specially the story that I abandoned a long time ago…**

 **I'd like to invite you all to comment and give your opinion of this story, good or bad...**

 **Now, this is more than obvious, but I think I'll say it anyway:**

 **ALL SONIC CHARACTERS ARE PROPERTY OF SEGA. UNDERTALE, ITS STORY AND CHARACTERS ARE PROPERTY OF TOBY FOX (and every person who helped him to create the game, we can't forget that).**

 **THIS STORY HAS NOTHING TO DO WITH ANY UNDERTALE AU: IT'S JUST A CROSSOVER.**

 **… And I guess this story concept has been used before, but I'll try to add my "special touch" (or whatever is called).**

 **... Well, I think that's all.**

 **Enjoy!**

* * *

 **CHAOSTALE**

 **Chapter 1: Once Upon a Time…**

* * *

A long time ago, there were two hedgehogs, **Sonic** and **Shadow** , who lived on Mobius.

They became enemies since their first encounter, due to their differences and the circumstances upon them. Yet, they became allies by fate; sharing the same speed and bold determination, instead of friends, they became eternal rivals.

Known as mighty rivals and heroes, their names echoed across their homeland.

 ***Day and night, light and dark, hero and antihero: that's how different they were.**

Despite all the struggles they have faced together, they walked down through different paths. And although those paths crossed more than once, it was just an instant, before they split once more.

One day…

Doctor Ivo Robotnik, well known as Eggman, announced his latest scheme.

This time wasn't about another attempt of world domination, but the announcement of his retirement.

Swearing on his own name, he communicated to the entire planet about his last project: an interdimensional machine that was able to make connection with other worlds, even beyond their own universe, with the only purpose to " _find a place where he could live in peace_ ".

Unable to trust in the words of their enemy, the heroes of Mobius went without delay to the doctor's lair.

Dancing between sabers, missiles and lasers, Sonic and their allies made their way through the base, their target being the machine located at the heart of the fortress. It didn't take long when they knocked down the doors of the chamber, and he was there: the doctor with a robot army and an improved version of the Egg Dragon.

Nothing that these brave heroes couldn't be able to handle.

Destroying robots, taunting Eggman and beating them all in a flash: for Sonic, it was like any other day...

 ***Until the doctor activated the machine.  
**  
Powered by the energy of the Chaos Emeralds, the machine opened a door to another dimension. However, it started to malfunction, transforming the harmless portal into a vortex that threatened to swallow everything in front of it; boxes, documents and scraps of metal were sucked through the portal, while the heroes and the doctor clung onto this world...

 ***Everyone, except Shadow the Hedgehog**

Nobody has noticed that the agent has stood motionless in front of the machine until that very moment. And when he freed himself from the trance he fell victim, it was too late:

Eggman wasn't willing to lose once more to his enemies, so, in a desperate attempt to make one of them fall, he ordered to the still standing robots to tackle Shadow into the abyss. In other circumstances, he would have been able to face all of Eggman's servants, but, being at the very edge of all reality, he could only stood firmly against them.

Everything happened so quickly:

Sonic, refusing to stay in crossed arms, used a Spin Dash to take down most of the robots, being helped by Silver, who used his powers as a life saver while Knuckles used his namesakes to get a hold of the wall and approach the machine at a steady pace.

The blue hero threw himself without hesitation to save his rival, among the roar of the machinery and the chaos in the chamber

However… This only ended with both of them falling into the unknown.

They both heard their friends calling out for them, just before they disappeared from Mobius.

...

The portal from where they fell slammed shut and they found themselves maneuvering in a spiral of colours and darkness; the cobalt hedgehog clung to his partner, watching as windows to other worlds opened and closed before their eyes, showing scenarios they could not catch sight.

Sonic noticed that the objects that has fallen with them started to vanish, this, caused by fragments of light that embedded and devoured them without a trace. To his own horror, those things were starting to stick onto their bodies the more they descended.

Pain coursed through their bodies; **light dissolved mass and readjusted cells, even from the inside.** As their bodies decreased in size, they feared that they would be reduced to nothing.

It hurt… The feeling never faded. On the contrary: it intensifies the more the light embraced them.

So unbearable was their torment that they lost the hold of each other...

Soon enough, they fell, into the darkness and unconsciousness. Colours died as they fell into the rabbit hole, heading into a Wonderland…

 ***Filled with magic and monsters.**

* * *

 **CHAOSTALE**

 **...**

 _Long ago,  
Two hedgehogs  
Fell into the world below._

 _By kindness_

 _Or hatred,  
Will they help us  
Or they'll FIGHT us?_

We know  
That we may not seem like it  
But, believe...  
That we are not your enemies.  
We just want to be free.  
But if we can't convince you, then,  
We cannot be friends...  
For the SOUL that you two protect  
It is the only key.

You don't have to be afraid,

 _I promise I'll give you aid._

 _If you keep your hopes and dreams,_

 _Maybe you'll find your way thorough._

 _Could it be they'll leave in peace?  
Or will they harvest their SOULS?..._

 _..._

Yalp ot ydaer uoy era?

* * *

He fell…

He fell into the deep, towards a golden sea that shone with the sun, and with no other company than the lurking shadows. Time slowed as he descended, welcomed into a peaceful slumber by the petals of the flowers below.

… When he awakened, he took on a deep breath, trying to escape from the realm of dreams.

Emerald orbs scanned their surroundings, hoping to find a familiar face or a hint of his whereabouts. However, he just found himself surrounded by golden flowers and green stems.

Ignorant of the path he should take, he followed a straight line, thinking that he may arrive somewhere, sooner or later.

He ran… or at least, tried to.

The flower seemed to reach to the sky, so it was necessary to move them aside with his hands, like jungle foliage. That wasn't all: he tripped more than once thanks to the vines that were growing all around the place. It seemed as if the beautiful flowers were laughing at him and his predicament.

And it didn't matter how much he kept moving forward…

 ***He was trapped in the deep of this hell hole.**

* * *

 **Hum… yeah. Those lines after the story's name are supposed to be a song.**

… **Welp! See ya next time (hopefully)!**


End file.
